1. Field of the Invention
The present invention relates to an exposure device that is included in an image forming apparatus such as a copier, a printer, and a facsimile and that includes a plurality of light sources linearly arranged for exposing an exposure object such as an image carrier to light. The present invention also relates to the image forming apparatus including the exposure device.
2. Description of the Related Art
In recent years, exposure devices that expose the surface of an exposure object such as an image carrier as a photosensitive body to light based on light information according to information of images to be formed are known for use in image forming apparatuses such as copiers, printers, and facsimiles.
The types of exposure devices include: a first type including an optical scanner in which a light source and a polariscope such as a polygon motor are combined; and a second type including an array light source device that includes a print head collectively exposing the surface of an exposure object to light in a main-scanning direction using a light emitting diode (LED) array or an organic electroluminescent (EL) array that includes a plurality of light sources linearly arranged and a rod lens array, as disclosed in, for example, Japanese Patent Application Laid-open No. H11-254737.
The advantages of the second type against the first type include: (1) having a small size; (2) having a small beam diameter to be written on an exposure object; and (3) having a long service life. A small exposure device contributes to downsizing of an image forming apparatus. A small beam diameter contributes to higher image quality. A long service life contributes to a longer service life of an image forming apparatus and recycling of the exposure device.
A print head in the exposure device of the second type having a structure using an LED array as the print head is produced by connecting a plurality of LED array chips. Therefore, an exposure feasible width on an exposure object corresponding to a plurality of light sources in an array direction varies depending on each print head due to a mounting error of LED array chips or similar defects. As a result, an image forming width varies depending on each exposure device. In an image forming apparatus that includes a plurality of exposure devices and forms color images, when exposure feasible widths of exposure devices are different from each other, a color shift occurs during color image formation thus lowering the image quality.
This color shift similarly occurs in an exposure device of the second type having a structure using an organic EL array as the print head. This is because the exposure feasible widths also vary depending on each print head due to the influence of a producing error of the organic EL array.
Examples of measures against such problems include correction of the exposure feasible widths by adding or deleting image data used for driving the print head in the array direction of a plurality of light sources. However, when the image data is simply deleted or added, image disturbance occurs at a portion where the addition or deletion is performed along a moving direction of an exposure object, for example, a rotation direction of a photosensitive body to generate a so-called vertical streak on the output image. The vertical streak is easily noticeable particularly on a halftone image.
For example, Japanese Patent Application Laid-open No. H11-254737 discloses a technique in which each pixel includes a plurality of LED elements as measures against vertical streaks arising from fluctuation in a light amount of each LED element.
However, even when such a technique is used, vertical streaks are still generated by adding or deleting image data used for driving a print head in the array direction of a plurality of light sources in order to correct the exposure feasible widths.
Moreover, for making such an exposure feasible width of each print head uniform to be a level in which a color shift caused during color image formation becomes unnoticeable, a manufacturing technique and a mounting technique with an extremely high precision are required, or the yield is lowered, which drives up costs.